Methods for the production of glass substrate blank

ABSTRACT

Provided are a method for producing a glass suitable for an information recording medium required to have high qualities, a process for the production of a glass substrate blank from the above glass and a process for the production of a glass substrate from the above blank. The method is for the production of a glass containing an alkali metal element by melting a glass material in a glass melting apparatus and uses, as said melting apparatus, a melting apparatus whose glass-contact portion is made of a material containing zirconium and containing substantially no alkali metal element. In the process for the production of a glass substrate blank, the glass obtained by the above method is press-molded or is subjected to a float process, and in the process for the production of a glass substrate, the above glass substrate blank is lapped and polished.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for the production of a glasssuitable for an information recording medium that is required to havehigh qualities, a process for the production of a glass substrate blankand a glass substrate from the glass obtained by the above productionmethod, an information recording medium to which the above glasssubstrate is applied, and a glass melting apparatus for use in the aboveglass production method.

2. Prior Art of the Invention

Generally, as a material for a glass-contact-portion of a glass meltingapparatus (to be sometimes referred to as “furnace material”hereinafter), an AZS (Al₂O₃—ZrO₂—SiO₂-containing) electrocast refractorymaterial is used for a high-temperature portion and analumina-containing electrocast refractory material is used for arelatively low-temperature portion. These furnace materials are desiredfor reasons that they cause less contamination to a glass and causesfoaming to a less degree. For example, these materials are used in aglass-contact-portion of a melting apparatus for soda lime glass, aglass for CRT, an optical glass and a glass for electronic devices. Forthe same reason, it is considered desirable to use an AZS electrocastrefractory material as a furnace material in a glass-contact portion fora glass as a material for a substrate of an information recording mediumsuch as a glass substrate for a magnetic disk.

Meanwhile, it has been mainstream practice to use an aluminum substrateas a substrate for a conventional magnetic disk. With increasing demandsfor a higher density of a magnetic disk, the substrate material for themagnetic disk is shifting from an aluminum material to a glass materialsince the glass material has a high Young's modulus and can easilyaccomplish the flatness of a disk surface. Further, with an increase inthe density of a magnetic disk, the distance between a recording mediumand a write/read head decreases markedly. These days, the above distanceis as close as a distance of approximately a few molecules arranged sideby side. In operation of a magnetic disk, it is required to turn thedisk several thousand times a second while maintaining the above finedistance as a distance between the head and the disk. The above state issomething like a state in which a large jetliner flies tens centimetersabove a ground without colliding with any obstacle, and the fact is thatthe surface of a substrate for an information recording medium isrequired to be free of any fine projection.

One of the substrate surface defects that hamper the abovehigher-density recording is a ZrO₂ mound problem. It is assumed that theZrO₂ mound is caused as follows. ZrO₂ has a higher hardness than glass.When the surface of a glass substrate containing ZrO₂ particles ispolished, the ZrO₂ particles present in the surface decrease slowly andthe glass decreases rapidly. As a result, ZrO₂ particles present in thesurface come to appear as fine projections on the substrate surface.These fine projections are zirconia mounds, and when the projections arepresent, forms of such projections are reflected on the surface of arecording medium and the projections cause a head crash that is acollision of a head with the surface of the magnetic disk. Inhigh-density recording media provided these days, even the presence ofonly one fine ZrO₂ mound causes the recording media to be defective.When a glass as a material for a substrate for an information recordingmedium is melted, therefore, it is required to perfectly prevent theinclusion of ZrO₂ particles.

There are large glass demands in the above field, and under thecircumstances, such demands can be fulfilled only when glass ismass-produced. There is therefore required a technique for stablyproducing a large amount of a glass containing no ZrO₂ particles.

When a glass as a material for a substrate of the above informationrecording medium is melted in a glass melting apparatus made of an AZSelectrocast refractory material, the ZrO₂ mound problem is liable to becaused. It has been considered that the above problem is caused asfollows. When the ZrO₂ content in a glass is large, for example, 5% byweight or more, ZrO₂ cannot be melted well, so that a non-meltedsubstance of ZrO₂ is liable to be generated. Therefore, attention hasbeen paid only to the content of ZrO₂, and no attention has been paid tothe relationship between a melting apparatus and the generation of aZrO₂ crystal. Under the circumstances, it has been impossible to obtaina large amount of glass substrates that contain no ZrO₂ crystal grainsand are free of ZrO₂ mounds.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a method for theproduction of a glass that is free of a surface fine projection problemcaused by high melting substances (ZrO₂, SiO₂, Al₂O₃, etc.) such as ZrO₂mounds and which can be suitably used as a substrate for an informationrecording medium.

It is another object of the present invention to provide a process forefficiently producing a glass substrate blank and a high-quality glasssubstrate from the glass obtained by the above method, an informationrecording medium to which the above glass substrate is applied, and aglass melting apparatus for use in the method for the production of aglass free of the above problem of ZrO₂ mounds, and the like.

For achieving the above objects, the present inventors have madediligent studies and as a result have found the following.

It has been found that it is neither because a ZrO₂ material as a glasscomponent is sparingly soluble nor because a glass composition contains5% by weight or more of a ZrO₂ component.

The mechanism of generation of ZrO₂ mounds is possibly based on areaction between Li element in glass components and Na element in aglass phase which is present in an AZS electrocast refractory materialconstituting a portion of contacting a high-temperature glass in amelting apparatus. In the AZS electrocast refractory material,approximately 1 to 10%, based on the weight of the refractory material,of Na₂O is present as a component in a glass phase. Therefore, Na ionfrom Na₂O contained in the glass phase of the AZS electrocast refractorymaterial and Li ion from Li₂O as a glass component undergo anion-exchange, and Na in the glass phase is substituted for Li, to causesoftening and to destroy the AZS crystal structure. As a result, the AZSrefractory material gets out of shape to form fragments and suchfragments are included in a molten glass in contact with the AZSrefractory material. ZrO₂ constituting the glass phase of the refractorymaterial is included in the glass, so that a number of ZrO₂ crystalgrains having a high melting point come to be present in the moltenglass.

On the other hand, a zirconia refractory material (having a ZrO₂ purityof at least 90%) contains almost no or little Na component, so that thecontact surface of the refractory material to a glass continues to besmooth and retains an original shape. As a result, a glass obtained bymelting it in such a refractory material shows no defects such asdevitrification and impurities.

On the basis of the above facts, when a glass containing Li element ismelted, the Na₂O impurity concentration in an electrocast refractorymaterial to be used is required to be 1% by weight or less, desirably,0.5% by weight or less.

As is clear from the above experimental results, a combination of aglass containing Li element and a melting apparatus formed of anelectrocast refractory material in which a glass phase contains Naelement causes the generation of ZrO₂ crystal grains in a glassmaterial.

The present inventors have therefore arrived at a conclusion that theuse of an alkali-free electrocast refractory material as a material fora apparatus for melting a glass containing Li element is essential forthe production of a glass material such as a material for a magneticdisk substrate that is required to have a surface flatness.

The present invention has been completed on the basis of the abovefinding.

That is, the present invention provides;

(1) a method for the production of a glass containing an alkali metalelement by melting a glass material in a glass melting apparatus, themethod using, as said melting apparatus, a melting apparatus whoseglass-contact portion is made of a material containing zirconium andcontaining substantially no alkali metal element,

(2) a method according to the above (1), wherein the glass containing analkali metal element is a glass containing lithium element,

(3) a method according to the above (1), wherein the material formingthe glass-contact portion of the glass melting apparatus is a materialcontaining substantially no sodium element,

(4) a method according to the above (1), wherein the glass containing analkali metal element is a glass containing at least one oxide selectedfrom SiO₂, Al₂O₃ and ZrO₂,

(5) a method according to the above (1), wherein the glass is a glass tobe chemically strengthened and/or to be crystallized,

(6) a process for the production of a glass substrate blank, whichcomprises press-molding the glass obtained by the method recited in theabove (1) in a molten state, or subjecting said glass in molten state toa float process,

(7) a process for the production of a glass substrate, which compriseslapping and polishing the glass substrate blank produced by the processrecited in the above (6),

(8) a process for the production of a glass substrate according to theabove (7), wherein the glass substrate is chemically strengthened afterthe lapping and polishing,

(9) a process for the production of a glass substrate according to theabove (7), wherein the glass is crystallized before the glass is finallypolished,

(10) a process for the production of a glass substrate according to theabove (7), wherein the glass substrate is a substrate for an informationrecording medium,

(11) an information recording medium comprising the glass substrateproduced by the process recited in the above (10) and an informationrecording layer formed on a main surface of the glass substrate, and

(12) a glass melting apparatus for melting a glass material to produce aglass containing an alkali metal, wherein a material of a portion thatcomes in contact with a glass when the glass material is melted isformed of a material containing zirconium and containing substantiallyno alkali metal element.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows a plan view, a front view and side views of a glass meltingvessel in Example 1.

PREFERRED EMBODIMENT OF THE INVENTION

The method for the production of a glass, provided by the presentinvention, will be explained first.

In this method, when a glass material is melted in a glass meltingapparatus to produce a glass containing an alkali metal element, thereis used a melting apparatus whose glass contact portion is made of amaterial containing zirconium and containing substantially no alkalimetal element.

In the glass substrate such as a glass substrate for an informationrecording medium, generally, the glass substrate is chemicallystrengthened for improving the glass substrate in strength. As a glassfor the above chemical strengthening, a glass containing an alkali metalelement, particularly, lithium element, as a glass component, is used.

The inclusion of crystal grains in a molten glass takes place since thefurnace material is corroded by the above ion-exchange between an alkalicomponent in the glass and an alkali component contained in the glassphase of furnace material of the glass contact portion of a meltingapparatus. That is, Li₂O in the glass is a component that undergoesion-exchange with sodium ion of the glass phase of the furnace materialduring melting, so that it is required to use an alkali-free material asa furnace material. The object of the present invention can beaccomplished by using a furnace material containing no alkali metalcomponent for the entire portion with which a glass comes in contactduring melting of the glass (to be referred to as “glass-contactportion” hereinafter). In a particularly preferred embodiment, theentire glass contact portion is formed of a furnace material made of aZrO₂ electrocast refractory material except for a heating electrode.

As a material for a substrate for an information recording medium, acrystallized glass has excellent properties as well. For the field of asubstrate for an information recording medium where a smooth mainsurface is demanded, crystallization is carried out while strictlycontrolling the size and density of a crystal phase. For precipitating acrystal phase containing, as a component, an alkali metal such aslithium, a glass (matrix glass) that is a starting glass for a crystalglass naturally contains an alkali metal. When the above glass ismelted, there is also caused a problem that the molten glass iscontaminated by the corrosion of a furnace material. When crystal grainsfrom the above contamination are included in the matrix glass, thecrystal grains appear as abnormal projections on a substrate surfaceeven if the above controlling is performed in the crystallization step,and the smoothness and flatness of a substrate surface are impaired. Formelting a matrix glass for a crystallized glass containing an alkalimetal, the method of the present invention is effective as well.

The above furnace material containing substantially no alkali metalcomponent refers to a furnace material containing an alkali metalcomponent in such an amount that the furnace material is not corroded byion-exchange with an alkali metal ion in a molten glass and a furnacematerial containing no alkali metal component. In the former case, thetolerable content of an alkali metal component differs depending uponconditions such as a melting temperature, a glass composition and aglass material. Generally, the corrosion of the furnace material can beprevented so long the content of the alkali metal component is 1% byweight or less. The above content is preferably 0.5% by weight. Afurnace material containing no sodium element is particularly preferred.

The above furnace material includes a refractory material containingmonoclinic zirconia and a glass phase combining monoclinic zirconia, andan electrically cast refractory material containing zirconia as a maincomponent (component whose content is the largest) and furthercontaining SiO₂. The electrocast refractory material can be obtained bycompletely melting a refractory raw material, casting a melt into amold, molding a cast melt and cooling a resultant product to solidness.

The method for the production of a glass, provided by the presentinvention, is suitable for producing a glass containing an alkali metalelement, particularly, a glass containing at least one oxide selectedfrom SiO₂, Al₂O₃ and ZrO₂ and containing an alkali metal element. Whenthe above alkali metal component is a component containing lithiumelement such as Li₂O, a particularly remarkable effect can be producedas compared with any other conventional method. When the content ofZrO₂, Al₂O₃ and SiO₂ in a glass is originally small, it is assumed thatZnO₂, Al₂O₃ and SiO₂ from the furnace material is possibly dissolved inthe glass, and crystal grains due to these substances from the furnacematerial are not generated. In a glass containing large amounts of suchcomponents, the dissolving reaction of the above substances from thefurnace material does not easily proceed, and as a result, fine crystalgrains are generated.

Therefore, the present invention is suitable for producing analuminosilicate glass, and particularly suitable for producing analuminosilicate glass containing ZrO₂.

Specifically, the above glass includes glasses containing 50 to 70 mol %of SiO₂, 0 to 20 mol % of Al₂O₃ and 0 to 15 mol % of ZrO₂ and having analkali metal oxide total content of 0.5 to 25 mol %. Of these, a glasscontaining Li₂O as an alkali metal oxide is more preferred, and a glasscontaining 0.5 to 20 mol % of Li₂O is still more preferred. Further, aglass containing 10 mol % or less of CaO or MgO which is a divalentcomponent, as an RO component, or a glass containing no such componentas a glass component is desirable.

SiO₂ is a main component for forming a glass network. When the contentthereof is less than 50 mol %, the liquidus temperature of a glass islow and the viscosity thereof is also low, so that it is difficult tomold the glass. When the content of SiO₂ exceeds 70 mol %, the viscosityof a glass is extremely high, so that it is difficult to melt the glass.The content of SiO₂ is therefore preferably in the above range.

Al₂O₃ is incorporated for improving a glass in the capability ofion-exchange in a glass surface. When the content thereof exceeds 20 mol%, there may be caused a drawback that a non-melt is generated due todeterioration of meltability. The content of Al₂O₃ is thereforepreferably in the above range.

ZrO₂ is a component that is not easily dissolved in a glass and isliable to form fine crystal grains. However, it is a component thatimproves a glass in chemical durability, strength and hardness as asubstrate and ion-exchange efficiency. However, when the content thereofexceeds 15 mol %, it is difficult to melt the glass. Therefore, thecontent of ZrO₂ is preferably in the above range.

Alkali metal oxides are components that chemically strengthen a glasswhen they undergo ion-exchange in a glass surface portion with alkalimetal ion in an ion-exchange bath. When the total content thereof isless than 0.5 mol %, it is not sufficient for strengthening of a glassby chemical strengthening. When it exceeds 25 mol %, the chemicaldurability of a glass may sometimes decrease. Therefore, the totalcontent of the alkali metal oxides is preferably in the above range.

Of the alkali metal oxides, Li₂O is a component that undergoesion-exchange in a glass surface portion mainly with Na ion in anion-exchange bath to chemically strengthen a glass. When the contentthereof is less than 0.5 mol %, the Young's modulus of the glass is low.When it exceeds 20 mol %, the chemical durability may be caused todecrease. Therefore, the content of Li₂O is preferably in the aboverange.

Examples of preferred glass compositions will be further given below.

A glass containing, by % by weight, 60 to 75% of SiO₂, 5 to 18% ofAl₂O₃, 4 to 10% of Li₂O, 4 to 15% of Na₂O and 3 to 15% of ZrO₂ and,particularly, having a Na₂O/ZrO₂ weight ratio of from 0.5 to 2.0 and anAl₂O₃/ZrO₂ weight ratio of from 0.4 to 2.5 (Glass 1).

A glass containing, by mol %, 35 to 65% of SiO₂, 0.1 to less than 15% ofAl₂O₃, 4 to 20% of Li₂O, 0 to 8% of Na₂O, the total content of Na₂O andLi₂O being 3 to 30%, 0.1 to 30% of TiO₂ and 1 to 45% of CaO, the totalcontent of MgO and the above CaO being 5 to 40% (Glass 2).

A glass containing, by mol %, 50 to 70% of SiO₂, 1 to 30% of Al₂O₃, 1 to20% of Li₂O, the alkali metal total content (Li₂O+Na₂O+K₂O) being 1 to25%, 0 to 10% of CaO+MgO as a total, 0 to 5% of ZrO₂ and 0 to 4% of TiO₂(Glass 3).

The glasses 1 and 2 are suitable for chemical strengthening, and theglass 3 is suitable as a matrix glass for a crystallized glass.

In the melting of the above glass, predetermined amount of oxides,carbonates, nitrates, hydroxides, etc., are weighed and mixed to obtaina formulated material, the formulated material is poured into a meltingapparatus heated at a temperature of from 1,150 to 1,600° C., preferably1,200 to 1,500° C., melted, and a melt is clarified and stirred tohomogenize it, whereby a molten glass is obtained. The molten glass isshaped in a desired form, to give a glass in which the total amount ofSiO₂, Al₂O₃ and ZrO₂ is contained in a molten state, that is, the abovecomponents are all contained in an amorphous state.

When a glass containing zirconium is melted, desirably, a glass materialcontaining zirconium is milled to a power state and supplied to amelting apparatus, since the glass material containing zirconium (e.g.,ZrO₂) has relatively poor solubility. For improving the glass materialin solubility, the above glass material preferably has a smallerparticle diameter on one hand. On the other hand, when the particlediameter of the glass material is too small, part of the glass materialis liable to fly off during preparation of the glass material or whenthe glass material is supplied to a melting apparatus. When part of theglass material flies off, the formulation ratio of the glass materialformulated to obtain a desired glass composition changes, and,undesirably, there is caused a deviation from the glass composition foran end product. The particle diameter of the glass material is thereforeadjusted to a range that is determined by taking account of thesolubility of the glass material and easiness in handling of the glassmaterial. When the glass material contains zirconium, the particlediameter of the glass material is preferably in the range of from 5 to50 μm for the above reason.

The melting apparatus used for melting the above glass has been alreadyexplained.

It can be confirmed visually, through a microscope or by irregularreflection of monochromatic light whether nor not the total amount ofSiO₂, Al₂O₃ and ZrO₂ is contained in a molten state.

The processes for the production of a glass substrate and a glasssubstrate, provided by the present invention, will be explained below.

In the process for the production of glass substrate blank in thepresent invention, a glass in a molten state produced by the abovemethod is press-molded or is subjected to a float process to produce aglass substrate blank. When a high-quality glass substrate for aninformation recording medium is produced at high yields, it is desirableto press-mold a molten glass produced by the above method in a statewhere the molten glass has a temperature at which it can bepress-molded, to produce a glass substrate blank having a form similarto a glass substrate. After annealed, the thus-prepared blank is lappedand polished to the form of a substrate, to produce a glass substratehaving a flatness and a smoothness to a remarkably high degree. Forimproving the glass substrate in strength as required, further, thesubstrate may be chemically strengthened or heat-treated forcrystallization.

In the production of the glass substrate blank, a predetermined amountof a molten glass obtained by melting and homogenization according tothe above method is supplied onto a molding surface, preferably onto amolding surface of a lower mold member under heat at a predeterminedtemperature, through a flow pipe. The molding surface has a powdery moldrelease agent applied thereto. Before the temperature of the glass comesto be lower than a press-moldable temperature range, the glass ispress-molded in the form of a glass substrate blank with an upper moldmember and the lower mold member or with the above upper and lower moldmembers and a sleeve. Then, when the molded product has a temperaturearound its glass transition temperature, the molded product is taken outof the mold. The molded product is quenched to a temperature around thedistortion point of the glass, and then annealed in an annealingfurnace, to give a glass substrate blank. In the course after thepress-molding and before the molded product is taken out, the moldedproduct may be pressed for correcting its warpage. In the supply of themolten glass onto the lower mold member, it is essential to adjust theglass temperature to a temperature range in which the glass cancontinuously flow out of the flow pipe and does not devitrify. Thethus-obtained glass substrate blank has the same composition as that ofthe above molten glass and contains no ZrO₂ crystal grains or containsnone of SiO₂ and Al₂O₃ crystal grains. While the process for theproduction of a substrate blank has been explained with reference to apress-molding method as an example hereinabove, the substrate blank maybe formed by a float process, or other known molding method such as adown draw molding method may be employed. In any molding method, it isdesirable to prevent crystallization caused by the devitrification of aglass during the molding and the treatment after the molding.

The thus-obtained glass substrate blank is lapped and polished tocomplete a glass substrate. The lapping and polishing can be carried outaccording to known methods. The glass substrate blank of the presentinvention contains no crystalline structure of ZrO₂ or ZrSiO₄. The abovecrystalline structure has a higher hardness than a glass having anamorphous state. If the substrate blank contains the above crystallinestructure, the polishing rate to the amorphous structure is much higherthan the polishing rate to the crystalline structure due to a differencein hardness when the main surface of the substrate is formed by lappingand polishing, so that projections of the crystalline structure areliable to be formed on the substrate surface. Since, however, thesubstrate blank of the present invention does not contain the abovecrystalline structure, the glass substrate obtained has no suchprojections formed on its surface and is suitable as a substrate for aninformation recording medium having a high recording density.

The chemical strengthening treatment will be explained below. The lappedand polished substrate is immersed in a molten salt containing alkalimetal ion, and in this treatment, alkali metal ion in the glasssubstrate and alkali metal ion in the molten salt undergo ion-exchange.In this case, the alkali metal ion to be contained in the molten salt isselected from alkali metal ion having a larger ionic radius than thealkali metal ion of the glass that is to undergo the ion-exchange. Forexample, when the glass substrate contains lithium ion, the molten saltpreferably contains sodium ion and/or potassium ion. When the glasssubstrate contains sodium ion, the molten salt preferably containspotassium ion. Since the glass substrate of the present inventioncontains lithium ion and sodium ion, it is preferred to use a moltensalt containing sodium ion and potassium ion. As a molten salt, it ispreferred to use a nitrate of an alkali metal, while a sulfate, hydrogensulfate, carbonate, halide, or the like may be used. For improving thechemical strengthening efficiency, the glass for constituting asubstrate preferably contains ZrO₂. When the glass contains ZrO₂, it isliable that ZrO₂ from a furnace material remains as a crystal phase inthe glass during the melting of the glass. In the present invention,however, no ZrO₂ from a furnace material is included even if a glasscontains ZrO₂ as a glass component, so that there can be obtained aglass substrate containing no ZrO₂ crystalline structure and thatchemically strengthened glass substrates can be produced at high yields.The chemically strengthened glass can be subjected to alkali elutiontreatment as required.

In the present invention, the glass may be crystallized to form acrystallized glass before final polishing treatment, and thecrystallized glass may be polished to produce a glass substrate.

The thus-obtained glass substrate has a remarkably flat and smooth mainsurface, and has no projections made of SiO₂, Al₂O₃ and ZrO₂ crystalstructures on the main surface. The glass substrate is thereforesuitable as a substrate for an information recording medium.

When the above substrate is used as a substrate for a magnetic recordingmedium, the magnetic recording medium surface and a write/read head donot come in contact, or collide, with each other even if the magneticrecording medium surface and the above head are brought close to eachother for complying with a high recording density, and the head canretain a stable flying height. There can be therefore provided a highlyreliable information recording medium. An information recording mediumcan be obtained from the above substrate by a known method in which aninformation recording layer (e.g., a magnetic layer when the informationrecording medium is a magnetic recording medium) is formed and,optionally, is coated with a protective layer for protecting therecording layer to form a multi-layer. The main surface of the aboveglass substrate for an information recording medium has a flatness of 20nm or less as a surface roughness (Ra).

When a substrate that is made of aluminosilicate glass containing ZrO₂and is chemically strengthened is used as the above substrate, thesubstrate has excellent weather resistance and excellent mechanicalstrength and has high reliability. Further, the substrate has highstability against the high-speed rotation of a disk-shaped informationrecording medium.

According to the present invention, there is also provided a glassmelting apparatus for melting a glass material to produce a glasscontaining an alkali metal, wherein a material of a portion that comesin contact with a glass when the glass material is melted is formed of amaterial containing zirconium and containing substantially no alkalimetal element.

EXAMPLES

The present invention will be explained more in detail with reference toExamples hereinafter, while the present invention shall not be limitedby these Examples.

Example

A glass melting vessel used in this Example is a vessel made of anelectrocast refractory material in which a glass material is poured andheated to obtain a molten glass.

FIG. 1 shows a plan view (i), a front view (ii) and side views [leftside view (iii) and right side view (iv)] of the glass melting vessel inthe Example. The melting vessel shown in FIG. 1 has a glass materialmelting portion on a front side (left hand side), into which a glassmaterial is introduced. The melting vessel has four side surfaces andone bottom surface, and has a material inlet port (a) on a front side(left hand side) and a molten glass outlet port (b) on a vessel backwardside (right hand side). The entire portion that comes in contact with amolten glass is formed of an electrocast refractory material containingno alkali metal.

Above the melting vessel, there are provided a bricked roof (not shown),a heating combustion burner (not shown) and a gas discharge port. Asshown in FIG. 1, a glass material is intermittently or continuouslycharged into the melting vessel through a portion (a) on the front sideof the melting vessel. The heating can be carried out by a method inwhich a mixture of a combustible gas (butane, propane, or the like) witha combustion aid gas (air, oxygen, or the like) is combusted above aglass liquid surface with a burner to melt the glass material, or amethod in which an electrode (SnO₂, MoO₂, or the like) for applyingelectricity is brought directly into contact with a molten glass to heatthe molten glass, or a combination of these methods. The glass melted bythe above heating method flows from (a) to (b) while it comes in contactwith a refractory material containing no alkali metal. The portion (b)is an outlet of the glass, and overflowed glass successively flows, forexample, to a clarification vessel and a working vessel in which amolten glass is homogenized by stirring, and the like. In theclarification vessel, a portion with which the molten glass comes incontact can be made of a refractory material containing no alkali metallike the melting vessel, or made of platinum or a platinum alloy. Theworking vessel can be constituted of platinum or a platinum alloy. Inthe above melting vessel, the molten glass may be clarified as well.

In the melting vessel shown in FIG. 1, generally, the level of a glassliquid surface is approximately 60 to 80% of the height of the sidewall,while the level may be lower than such a percentage.

While this Example uses a refractory material containing no alkali metalfor forming the portion that comes in contact with a molten glass, theremay be also used a commercially available refractory material of whichthe alkali metal content is limited to a very low level (1% by weight orless, preferably 0.5% by weight or less). The above refractory materialincludes, for example, an electrocast refractory material in which 94%by weight of monoclinic zirconia is dispersed in 6% by weight of a glassphase. The content of Na₂O in the refractory material is limited to 0.3%by weight or less, and some refractory materials contain small amountsof SiO₂, Al₂O₃ and TiO₂ in addition to main components.

A glass material that was to give a molten glass 1 containing SiO₂,Al₂O₃, Li₂O, Na₂O and ZrO₂ was melted in the above melting apparatus. Aglass material that was to give a glass 2 containing SiO₂, Al₂O₃, Li₂O,Na₂O, TiO₂, CaO and MgO was melted in the above melting apparatus. Aglass material that was to give a glass 3 containing SiO₂, Al₂O₃, Li₂O,Na₂O, K₂O, CaO, MgO, ZrO₂ and TiO₂ was melted in the above meltingapparatus.

After the above glass materials were melted, no corrosion was found onthe refractory material which had been in contact with molten glassesfor a long period of time, and it was sufficient to perform maintenanceof the melting vessel at an ordinary level.

Information recording media were produced from the molten glasses 1 to 3by the following two molding methods (first and second molding methods).

The first molding method is as follows. A clarified and homogenizedmolten glass was allowed to flow through a flow pipe made of a platinumalloy at a constant speed, and the glass that was flowing out wasreceived on a mold (lower mold member) consecutively. Each receivedglass was molded into a thin-plate disk-shaped glass with the abovelower mold member and an upper mold member that faced the lower moldmember. The thin-plate glass was transported into an annealing furnaceand annealed. Such thin-plate disk-shaped glasses formed from the moltenglasses 1 and 2 were subjected to outer diameter and inner diameterprocessing and surface lapping and polishing, and then immersed in amolten salt of sodium nitrate and potassium nitrate for chemicalstrengthening, whereby disk-shaped glass substrates for an informationrecording medium were formed.

Thin-plate glass formed from the molten glass 3 was subjected to aseries of steps including outer diameter and inner diameter processing,surface lapping and polishing and crystallization by heat treatment, togive a crystallized glass substrate for an information recording medium.

None of the above thin-plate glasses and the above substrates hadsurface fine projections caused by contamination during the melting, andthey satisfied requirements of a substrate for an information recordingmedium.

In the second molding method, each of clarified and homogenized moltenglasses 1 to 3 was subjected to a float process to form thin-plateglasses. Each thin-plate glass obtained by the float-process wasannealed and then shaped into a disk. Thereafter, such disks weretreated in the same manner as in the first molding method, to givedisk-shaped substrates for an information recording medium. None of theabove thin-plate glasses and the above substrates had surface fineprojections caused by contamination during the melting, and theysatisfied requirements of a substrate for an information recordingmedium.

A multi-layer film including a magnetic recording layer was formed oneach of the above substrates, to give information recording media.

Since the portion of the melting vessel that comes in contact with amolten glass is formed of the furnace material containing substantiallyno alkali metal as described above, the molten glass contaminationcaused by inclusion of furnace materials can be prevented even if themolten glass contains an alkali component, and there can be efficientlyproduced, at high yields, an alkali component-containing glasscontaining no crystalline structure of a high melting substance, achemically strengthened glass formed of the above glass, and a substrateformed of a crystallized glass obtained by heat treatment of the aboveglass.

By using the above substrate, information recording media such as amagnetic disk can be stably produced efficiently at high yields.

Comparative Example

Each of the above glass materials used in the Example was melted in amelting vessel made of a refractory material containing ZrO₂ and Al₂O₃as main component and containing other components such as SiO₂, Na₂O andthe like. The above refractory material had an Na₂O content of 1.5% byweight. When the melting was continuously carried out, a portion thatcame in contact with molten glass was corroded. Each of the glassesmelted as above was clarified, stirred and supplied onto a mold througha flow pipe. A received glass was press-molded to form a substrate blankfor an information recording medium. The thus-obtained blanks wereannealed to remove their distortion, and the surface of each blank waslapped and polished to form substrates. When the substrate surfaces wereobserved, fine projections were found. The projections were analyzed toshow that they were formed of zirconia. These projections were to causehead clash, so that the substrates were not usable as substrates for anymagnetic disk.

EFFECT OF THE INVENTION

According to the method for the production of a glass, provided by thepresent invention, a glass containing an alkali metal element can beefficiently produced at high yields in a dissolved state in which nocrystalline structure of a high melting substance such as ZrO₂ or ZrSiO₄is present.

According to the present invention, there can be provided a meltingapparatus for efficiently producing a glass containing an alkali metalelement at high yields, a glass substrate blank containing nocrystalline structure made of a refractory substance, and a flat glasssubstrate having no surface projections made of fine crystallinestructures.

Furthermore, according to the present invention, there can be providedan information recording medium having a flat glass substrate having nosurface projections made of fine crystalline structures, and there canbe produced a highly reliable information recording medium that canattain a high recording density.

1. A process for the production of a magnetic recording medium, whichcomprises the steps of: (a) selecting a glass melting apparatus whoseglass-contact portion is made of an electrocast zirconia refractorymaterial having a ZrO₂ purity of at least 90% by weight percent and issubstantially free of sodium element, (b) melting a glass materialcontaining lithium element in the glass melting apparatus selected instep (a) to obtain a glass containing an alkali metal element therebyavoiding the generation of ZrO₂ crystal grains in the glass materialtherein, and (c) lapping and polishing the glass produced in the step toobtain the glass substrate having no or substantially free of ZrO₂mounds thereon and forming a magnetic layer on the lapped and polishedglass substrate to obtain the magnetic recording medium.
 2. The processaccording to claim 1, wherein the glass substrate is chemicallystrengthened after the lapping and polishing.
 3. The process accordingto claim 1, wherein the glass obtained containing an alkali metalelement is crystallized before the glass is finally polished.
 4. Theprocess according to claim 1, wherein the glass containing an alkalimetal element is a glass containing at least one oxide selected fromSiO₂, Al₂O₃ and ZrO₂.
 5. The process according to claim 1, wherein theglass obtained containing an alkali metal element is a glass to bechemically strengthened and/or to be crystallized.
 6. The processaccording to claim 1, wherein the material constituting a glass-contactportion of the glass melting apparatus contains 1% by weight or less ofNa₂O impurity.
 7. The process according to claim 6, wherein the materialconstituting a glass contact portion of the glass melting apparatuscontains 0.5% by weight or less of Na₂O impurity.
 8. The processaccording to claim 7, wherein the material constituting a glass contactportion of the glass melting apparatus contains 0.3% by weight or lessof Na₂O impurity.